DMRC JE Electrical Question Paper With Explained Solution – 2016

Switching sequence – to turn on or off a bus bar, equipment, power plant requires proper steps and skill. A single mistake would cause an accident.

Step 1: Open the circuit breaker: CB is a device that can make and break the circuit under normal as well as under faulty conditions manually or automatically.

Step 2: Open the isolator: An isolator is a device that always operates under no-load conditions. This is because it has no provision for arc quenching.

Its function is to isolate the circuit after operation of the circuit breaker and discharge the grapes charges to earth through the earth switch.

Step 3: Operate the Earth switch: After opening the circuit breaker and manually disconnecting the isolator the next is to engage the earth switch. Its function is to discharge any remnant charge due to residual magnetism which might be lethal to humans and expensive testing equipment.

The Thevenin voltage e used in Thevenin’s Theorem is an ideal voltage source equal to the open circuit voltage at the terminals. In the example below, the resistance R₂ does not affect this voltage and the resistances R₁ and R₃ form a voltage divider, giving

$\begin{array}{l}e = \dfrac{{{V_1}{R_3}}}{{{R_1} + {R_3}}}\\\\e = \dfrac{{10 \times 10}}{{10 + 10}} = 5V\end{array}$

The Thevenin resistance r used in Thevenin’s Theorem is the resistance measured at terminals AB with all voltage sources replaced by short circuits and all current sources replaced by open circuits. It can also be calculated by dividing the open-circuit voltage by the short circuit current at AB, but the previous method is usually preferable and gives

$\begin{array}{l}r = {R_2} + {R_1}||{R_3}\\\\ = {R_2} + \dfrac{{{R_1}{R_3}}}{{{R_1} + {R_3}}}\\\\ = 5 + \dfrac{{10 \times 10}}{{10 + 10}}\\\\ = 10\Omega \end{array}$

 

Hence the Thevenin’s Voltage is 5 V and the Thevenin’s Resistance is 10Ω

A long transmission line can be considered to be composed of a high amount of capacitance and inductor distributed across the entire length of the line. Ferranti Effect occurs when the current drawn by the distributed capacitance of the line is greater than the current associated with the load at the receiving end of the line which occurs during light or no load.

Shunt Reactor compensation at the receiving end help to reduce the effect of capacitance thus reducing the Ferranti effect.

The shunt Reactor absorbs the excess reactive power during no load or light load conditions and thus helps in stabilizing the voltage of the Transmission Line.

Since the resistor 10Ω and 15Ω are connected in parallel, therefore, its equivalent resistance

= (10 × 15) ÷ (10 + 15) = 6Ω

Now 6Ω and 12Ω are connected in series hence total resistance is

= 6 + 12 = 18Ω

When a conductor of resistance R_o at 0°C  is heated to t° C. Its resistance R_t after heating is given by

R_t =  R_o (1 + tα _t)

13 = R_o(1 + 50 × 0.006)

13 = 1.3 R_o

R_o = 13 ÷ 1.3 = 10Ω

Electric energy is the total amount of electrical work done in an electrical circuit. Electric energy can also be defined as the product of power and time. The S.I Unit of Electrical- Energy is joule or watt-sec.

1.5 kW = 1500 joule

time = 5 minute = 5 × 60 = 300 sec

Electric Energy = 1500 × 300 = 45,000 Joules

Emf is not a force at all; it is a special type of potential difference. To be precise, the electromotive force (emf) is the potential difference of a source when no current is flowing. Units of emf are volts.

If there is no load connected to the Voltage source, then no-load current will flow, and there will no internal voltage drop.

The voltage appearing across the voltage source’s terminals under this condition is called its open-circuit voltage or no-load voltage which exactly equals the electromotive force (e.m.f. of that source).

A voltage source open-circuit, or no-load, voltage corresponds to its electromotive force.

Full scale deflection 0.1 mA

Now current through 0.5 FSD = 0.5 × 0.1 = 0.05 mA

When one wattmeter reads negative during the measurement of power by two wattmeter method. The power factor of the load in such a case will be less than 0.5 lagging.

Wattmeter cannot show negative reading as it has only a positive scale. An indication of negative reading is that the pointer tries to deflect in a negative direction i.e. to the left of zero. In such a case, reading can be converted to positive by interchanging either pressure coil connections or by interchanging current coil connections. Remember that interchanging connections of both the coils will have no effect on wattmeter reading.

A unit of resistance, equal to that of a column of mercury of uniform cross-section that has a length of 160.3 centimeters and a mass of 14.4521 grams at the temperature of melting ice; it has been superseded by the ohm, and is equal to 1.00049 ohms.